It has long been known to provide computer workstations interconnected by digital communication networks whereby users of the individual workstations may communicate with one another over the network. Previously this was common, for example, by means of a typed note, data or program file transmitted to another user. More recently, users have increasingly requested desktop conferencing, remote presentations, and other multimedia applications between network users. However, such multimedia applications have associated therewith data-intensive sound, voice, and video flows. This requires concomitant high bandwidth communication links between distributed computing systems with minimal communication delay, maximum throughput, and instantaneous burst communication capability. The requirements of such multimedia applications accordingly make scheduling appropriate resources to provide for necessary quality of service very difficult.
Prior art has recognized that certain data in a network, such as that associated with multimedia, may require priority handling. Thus, for example, a "quality of service" (QOS) has been defined in the literature, hereinafter described in more detail. This seeks to describe various parameters which may be specified in an attempt to define certain minimum requirements which must be met for transmissions of given data types over the network. See, for example, quality of service standards set forth in the Open System Interconnect Standard X.214 of the International Standards Organization interface and the quality of service standards defined in CCITTQ.931 (ISDN), Q.933 (frame relay), and Q.93B (B-ISDN ATM) drafts.
As yet another example, there is an architected priority mechanism in the IEEE 802.5 Token Ring. A station on the ring with a high priority frame to send may indicate this in an access control field of a passing frame. When a station sending the frame releases the token, it releases the token at the priority of the AC field, and eventually sets it back to its original priority as specified in an IEEE 802.5 media access control (MAC) protocol. The IEEE standard and implementations thereof merely specify a protocol for increasing and decreasing priority, but each station is unconstrained in its use of priority beyond this protocol.
Several references have addressed the problem of priority traffic management in multimedia communication network systems. For example, the International Standards Organization (ISO) and International Telephone and Telegraph Consultative Committee have specified quality of service parameters as part of the link layer interface (CCITT X.212 and ISO 8886). ISDN provides a comparable standard, Q933. These parameters include throughput, transit delay, residual error rate and resilience to faults in the physical media to describe bandwidth reservation requirements. Token ring time division multiplexing schemes for propagating priority traffic across a token ring have been discussed in U.S. Pat. No. 4,843,606, "Local Area Communications Systems for Integrated Services Based on a Token Ring Transmission Medium", by Bux et al, and U.S. Pat. No. 4,539,679, "Synchronization in a Communication Network of Interconnected Rings", also by Bux et al.
Moreover, the IEEE 802.5 priority mechanism has been proposed for voice in the token ring, and prototyping has been performed of a network layer bandwidth manager which performs bandwidth reservation on links along a path, and implements end-to-end bandwidth reservation using the Internet experimental stream protocol RFC 1190, using token ring priority.
Notwithstanding the foregoing, several problems nevertheless remain which have not been effectively addressed by the prior art in providing for bandwidth for reserved multimedia traffic. One problem relates to the emergence of heterogeneous networks from differing vendor implementations of multimedia sessions. This requires that, in providing for reserved bandwidth connections, a solution must be provided which minimizes changes to application program interfaces and underlying client implementations. Relative to the problem of heterogeneous session herein above mentioned, it is typically not practical or possible to control what software applications a client puts on a ring or transmission. The customer will typically have applications on a ring which send frames at a predetermined size. Thus it is not feasible to constrain normally the average size frames sent by each station sending frames at a lower priority than the high priority multimedia server.
From the foregoing, it will be apparent that it is necessary to guarantee that multimedia session obtain at least a minimal amount of bandwidth to insure that sound, voice, and video can be delivered within a certain amount of time. On token ring communication networks, a priority scheme is employed for multimedia so that a station can make a reservation in a passing token ring frame and obtain a token after the frame is transmitted. In implementations of the token ring wherein network adapters release the token after each send, a single multimedia server may capture no more than fifty percent of the tokens. If there are file transfers occurring on the ring, each time the server releases the token, a station will capture it to send a data frame. However, if the frame size for the file transfer is equal to the frame size of the multimedia transfer, the server may obtain no more than half the ring bandwidth even when priority is employed. If the file transfer frame size is greater than the multimedia transfer size, the bandwidth allocated to priority multimedia traffic could become arbitrarily small.
It is desirable in such computerized network environments that systems be configurable such that a server, disk, client, transport and network subsystem obtains as much or as little resource reservation as is possible or desired. If the system is needed to support an absolute maximum number of multimedia sessions, such as video sessions, then some means is needed to protect the multimedia flows on the token ring from interference from unreserved traffic, such as normal file system activity. However, data frames on a token ring can typically be as large as 9.1 milliseconds in transfer time, e.g. over 16 Kbytes on a 16 Mbps token ring. Accordingly, it will be readily apparent that some means was needed to insure that data traffic having large frames did not consume more ring bandwidth than the system administrator configured for the multimedia traffic.